On the 1<i>A</i>1–3<i>B</i>1 separation in CH2 and SiH2

Abstract

We have determined the 1A1–3B1 separation (Te) in both CH2 and SiH2 using very large Gaussian basis sets (including g functions) and second-order CI wave functions. Complete geometry optimizations have been performed, and relativistic effects have been included using first-order perturbation theory. This treatment yields Te values for the 1A1–3B1 separation of 9.07 kcal/mol in CH2 and −20.58 kcal/mol in SiH2. Using a combination of theoretical and experimental values to estimate the contribution of zero-point vibration to the separation yields T0 values of 8.9 kcal/mol for CH2 and −20.9 kcal/mol for SiH2, in excellent agreement with the experimental values of 9.02 and −21.0 kcal/mol. A corollary to the small zero-point vibrational contribution to the separation is that the symmetric stretching fundamental in CH2(3B1) must be near 3100 cm−1, much less than a recently suggested value of around 3400 cm−1. Our accurate Te value for SiH2 establishes the ionization potential of the 1A1 state as 9.15 eV, the higher of two recent experimental values.